In ring oscillators and inductance-capacitance voltage-controlled oscillators (LC-VCO), temperature drift is often solved by using temperature sensing and compensation circuits. For example, typical VCO implementations may use a Proportional to Absolute Temperature (PTAT) circuit to counter temperature drift of oscillator cores. In another example, a silicon chip with on-die temperature sensors may use a temperature sensor code as a mechanism to compensate for temperature drift. In the above approaches, temperature compensation is an open-loop mechanism, thereby requiring careful device characterization and post-silicon trimming.
In contrast, a closed-loop approach might include detecting a control voltage of the oscillator. If the control voltage is too far from a preset operating point, an error signal can be fed back to adjust common-mode inputs of a varactor coupled to the LC-VCO, thereby forcing the control voltage to the preset voltage. This implementation in the analog domain, however, can be costly due to various reasons.
Another approach is to provide no compensation at all, in which case frequency coverage range of a fine-tuning curve is simply extended. In the case of a digitally controlled oscillator (DCO), however, this may mean longer fine-tuning word length, thereby potentially incurring higher logic power consumption.